This invention relates to a method for uphill casting in sand molds with high-resin casting cores, or in particular casting cores containing binders, and with directed solidification of metal castings with at least one cavity, particularly, casting of prototypes of engine blocks or cylinder heads, for example, for internal combustion engines that are provided with a cavity for the passage of coolant water.
Corresponding castings in the form of engine blocks or cylinder heads with channels for coolant fluid are made today in large numbers of light metal or aluminum or magnesium alloys. Such castings pursuant to the state of the art are made by casting liquid metal in chill molds, which produces good dimensionally stable surfaces. A general problem for casting production results from the necessary coolant water channels, which can be produced practically only by using casting cores. These casting cores are so-called lost cores, which have to be removed after the cooling of the block. They are usually made of mold compounds, for example of mold sand using binders. Another method for producing such blocks consists of making patterns out of polystyrene foam. During the casting of the liquid metal, the polystyrene foam melts and burns up. In both cases the gases formed have to be removed by suction. Furthermore, gas bubbles may form during the casting, which lead to gas defects and leaks in the finished casting.
According to DE 36 18 059 A1, the gases formed in the chill mold during uphill casting are drawn off through two ventilation ports during the filling, which are placed at the highest point of the casting mold. In this case the ventilation ports have to be so tight that the liquid metal cannot pass through. For the tight infeed of the casting, the casting pressure as a rule has to be maintained up to a given degree of solidification so that the shrinkage of the casting occurs in the area of the riser tube in the poured section of the mold.
This method cannot be carried over to the production of prototypes by means of sand molds. In the preparation of prototypes, high-resin laser-sintered casting cores that have high binder content are being used more and more frequently. These likewise cause gases during the casting that can bring about foam, pores, and bubbles on the surface of the casting. Because of later mass production in the automobile industry, high-performance prototypes also have to be made available, which guarantee a high quality standard, especially for high-load components.
For this reason, the present invention has an objective of developing a casting method, especially for prototypes, that provides castings with assembly line characteristics.
It is well known that casting cores are given ventilation holes at the core marks in order to carry off the casting gases from the casting core during casting. According to DE 24 26 717 A1, it is known that the air in the mold cavity is carried off by suction during the casting, and a partial vacuum is produced in the mold cavity. The air in the mold cavity can thus cause no counterpressure with the gases that are formed, so that the actual casting pressure is reduced, at least in the area of the outer wall of the later cast part. This can avoid gas defects, because the air can escape quickly and at the right time. The elevation of casting pressure depends on a corresponding increase of the specific pressure of the surrounding atmosphere acting through the mouth of the mold or the head of the casting. To produce a casting, a mold is used in which the perforated wall of the mold cavity is connected to the vacuum source through a pipe. The rate of filling with molten metal can thereby be increased. The same conditions thus exist in the method described in DE 22 58 461 A1 and DE 32 40 808, in which the casting molds are provided with air-permeable walls and are connected to a vacuum source.
However, increasing the rate of filling may cause turbulence in the molten metal, by which parts of the sand mold and slag are loosened and enclosed. To prevent this as much as possible, uphill casting has proved useful for mold casting, since the molten metal in this case is not made turbulent, but the mold is filled with a calm front of melt. Oxide inclusions in the casting can thereby largely be avoided.
The task of the invention thus consists of a method for uphill casting in which a low-turbulence flow of liquid molten metal is provided for while removing air from the casting mold. Hindrance to filling from pockets of air in the mold is also to be eliminated. At the same time, dense and pore-free mechanically strong castings are to be produced by the method.
This invention provides a method for uphill casting in sand molds with high-resin casting cores or especially casting cores containing binders, and with directed solidification of metallic castings that have at least one cavity. In particular, the method provides prototypes of engine blocks or cylinder heads, for example for internal combustion engines that are provided with a cavity through which coolant water flows.
To provide for low-turbulence flow of the liquid molten metal during the deaeration of the casting mold, the casting mold (1) comprises at least one feeder head (24) that is connected through a casting system (22) to an infeed funnel (23), and the molten metal is displaced by gravity through the casting system (22) into the casting mold (1), with the cavity (11xe2x80x2) in the molten metal being subjected after the filling to reduced pressure that is greater than the pressure of the core gases formed in the core.
This eliminates the hindrance to filling the mold from air pockets in the mold. At the same time, dense and pore-free, mechanically strong cast parts are produced by the method.
According to the invention, a method for uphill casting in sand molds is provided in which the cavity formed by the high-resin or binder-containing casting core is subjected after loading the molten metal to reduced pressure, which eliminates the pressure of the core gases relative to the molten metal, with the entry of core gas into the molten metal being prevented.
Pursuant to a preferred refinement of the invention, the reduced pressure in the cavity is generated by a vacuum device connected to at least one core mark. While the molten metal in known gravity casting flows into the casting mold under the influence of gravity and solidifies under normal atmospheric pressure, in the proposed method the molten metal solidifies under reduced pressure that is generated in the core.
According to another refinement of the invention, the removal of air from the casting mold is accomplished through the vacuum device connected to the core marks. Removing air from the casting mold through the core marks has the advantage that no reduced pressure can be created in the casting mold during casting that increases the rate of filling by molten metal. This can be attributed to the fact that the reduced pressure in the cavity can be created only when the casting core is completely enveloped by liquid molten metal and is enclosed air-tight by it. Thus air can be removed from the casting mold during the casting.
According to a further development of the invention, dense infeed can be improved if the casting mold is cooled during the venting of air. Cooling causes directed solidification of the molten metal, which will be explained in detail below with reference to an example of embodiment and with reference to drawings.